The present invention relates generally to a power source for an implantable medical device, and more particularly, the present invention relates to a dual cell power source for optimizing implantable medical device performance.
A variety of different implantable medical devices (IMD) are available for therapeutic stimulation of the heart and are well known in the art. For example, implantable cardioverter-defibrillators (ICDs) are used to treat those patients suffering from ventricular fibrillation, a chaotic heart rhythm that can quickly result in death if not corrected. In operation, the ICD continuously monitors the electrical activity of a patient's heart, detects ventricular fibrillation, and in response to that detection, delivers appropriate shocks to restore normal heart rhythm. Similarly, an automatic implantable defibrillator (AID) is available for therapeutic stimulation of the heart. In operation, an AID device detects ventricular fibrillation and delivers a non-synchronous high-voltage pulse to the heart through widely spaced electrodes located outside of the heart, thus mimicking transthoratic defibrillation. Yet another example of a prior art cardioverter includes the pacemaker/cardioverter/defibrillator (PCD) disclosed, for example, in U.S. Pat. No. 4,375,817 to Engle, et al. This device detects the onset of tachyarrhythmia and includes means to monitor or detect progression of the tachyarrhythmia so that progressively greater energy levels may be applied to the heart to interrupt a ventricular tachycaria or fibrillation. Numerous other, similar implantable medical devices, for example a programmable pacemaker, are further available.
Regardless of the exact construction and use, each of the above-described IMDs generally include three primary components: a low-power control circuit, a high-power output circuit, and a power source. The control circuit monitors and determines various operating characteristics, such as, for example, rate, synchronization, pulse width and output voltage of heart stimulating pulses, as well as diagnostic functions such as monitoring the heart. Conversely, the high-power output circuit generates electrical stimulating pulses to be applied to the heart via one or more leads in response to signals from the control circuit.
The power source provides power to both the low-power control circuit and the high-power output circuit. As a point of reference, the power source is typically required to provide 10–20 microamps to the control circuit and a higher current to the output circuit. Depending upon the particular IMD application, the high-power output circuit may require a stimulation energy of as little as 0.1 Joules for pacemakers to as much as 40 Joules for implantable defibrillators. In addition to providing a sufficient stimulation energy, it is desirable that the power source possess a low self-discharge to have a useful life of many years, and that it is highly reliable, and able to supply energy from a minimum packaged volume.
Suitable power sources or batteries for IMD's are virtually always electrochemical in nature, commonly referred to as electrochemical cells. Acceptable electrochemical cells for IMDs typically include a case surrounding an anode, a separator, a cathode and an electrolyte. The anode material is typically a lithium metal or, for rechargeable cells, a lithium ion containing body. Lithium batteries are generally regarded as acceptable power sources due in part to their high energy density and low self-discharge characteristics relative to other types of batteries. The cathode material is typically metal-based, such as silver vanadium oxide (SVO), manganese dioxide, etc.
In some cases, the power requirements of the output circuit are higher than the battery can deliver. Thus, it is common in the prior art to accumulate and store the stimulating pulse energy in an output energy storage device at some point prior to the delivery of a stimulating pulse, such as with an output capacitor. When the control circuit indicates to the output circuit that a stimulating pulse is to be delivered, the output circuitry causes the energy stored in the output capacitor to be applied to the cardiac tissue via the implanted leads. Prior to delivery of a subsequent stimulating pulse, the output capacitor is typically recharged, with the time required for the power source to recharge the output capacitor being referred to as the “charge time”.
Regardless of whether an output capacitor(s) is employed, one perceived drawback of currently known therapeutic pulsing IMDs is that they often have to be replaced before their battery depletion levels have reached a maximum. When an IMD's output capacitor is being recharged, there is a drop in battery voltage due to the charging current flowing through an inherent battery impedance. Although this voltage drop may not be significant when the battery is new or fresh, it may increase substantially as the battery ages or is approaching depletion, such that during a capacitor recharging operation, the voltage supply to the control circuit may drop below a minimum allowable level. This temporary drop can cause the control circuit to malfunction. The IMD may be removed and replaced before any such malfunctions occur, even though the battery may still have sufficient capacity to stimulate the heart. Simply stated, the rate capability of currently available lithium-based cells is highly dependent upon time or depth-of-discharge as the cell develops high internal resistance over time and/or with repeated use. For IMD applications, this time or depth-of-discharge dependence limits the battery's useful life.
One solution to the above-described issue is to provide two batteries, one for charging the output circuit or capacitor and a separate battery for powering the control circuit. Unfortunately, the relative amounts of energy required by the device for the control and charging/output circuitry tend to vary from patient to patient. The capacity of the battery to power the control circuit can only be optimized with regard to one patient profile. Thus for other patients, one battery may deplete before the other, leaving wasted energy in the device. An example of such a system is disclosed in U.S. Pat. No. 5,614,331 to Takeuchi et al.
An additional, related concern associated with IMD power sources relates to overall size constraints. In particular, in order to provide an appropriate power level for a relatively long time period (on the order of 4–7 years), the power source associated with the high-power output circuitry typically has a certain electrode surface area to achieve the high-rate capability. Due to safety and fabrication constraints, the requisite electrode surface area can be achieved with an increased cell volume. The resulting cell may satisfy output circuitry power requirements, but unfortunately may be volumetrically inefficient. Even further, recent IMD designs require the power source to assume a shape other than rectangular, such as a “D” or half “D” contour, further contributing to volumetric inefficiencies.
In general terms, then, currently available electrochemical cell designs, especially Li/SVO constructions, may satisfy, at least initially, power requirements for the output circuitry. The inherent volumetric inefficiencies of these cells, however, dictates an end-of-life point at which less than the cell's useful capacity has been used. Once again, currently available cells exhibit an output circuitry charge time that is highly dependent upon time of use or depth-of-discharge. Over time, the cell's impedance increases, thereby increasing the resulting charge time. Virtually all IMDs have a maximum allowable charge time for the output circuitry. Once the cell's charge time exceeds the maximum allowable charge time, the IMD may be replaced. The volumetrically inefficient cell may quickly reach this maximum charge time, even though a large portion of the cell's capacity remains unused (on the order of 40% of the useful capacity). Thus, regardless of whether the power source incorporates one or two cells, the resulting configuration is highly inefficient in terms of the high-rate battery's useful capacity.
Manufacturers continue to improve upon IMD construction and size characteristics. To this end, currently available power source designs are less than optimal. Therefore, a need exists for an IMD power source having superior space-volumetric efficiencies and a higher energy density, without a proportional increase in charge time.
Yet another issue associated with IMD power sources involves the use of a wireless transceiver to communicate IMD data with an external device. The data communicated by the IMD may include physiological data related to the patient in which the IMD is implanted. For example, if the IMD is a pacemaker or cardioverter/defibrillator, the physiological data may include electric cardiac signals obtained from electrodes implanted within the patient's heart as previously discussed. The external device with which the IMD communicates this physiological data may include a computer, for example, that monitors and/or processes the physiological data that is received from the IMD.
The IMD may also communicate data related to its performance, such as the intensity level in which it delivered a therapeutic shock for a given set of electric cardiac signals monitored via the implanted electrodes. The external computer device may analyze the received data and transmit programming data to the IMD to adjust its therapy. For example, the programming data may indicate to the IMD to reduce the intensity of the therapeutic shock delivered to the patient.
Typically, the wireless transceiver within the IMD requires relatively high current pulses, thus resulting in a higher drain from the power source within the IMD. As the sophistication of the IMD and the number of communication transmissions performed by the IMD is expected to increase over the next several years, a much higher burden may be placed on the IMD's power source, thus reducing its life. Because the accessibility of the power source is achieved typically via a surgical procedure, this reduction in battery life is a concern.
The present invention is directed to reducing the effects of one or more of the problems set forth above.